Refactor RenetClient so channels are accessed more efficiently

[UkoeHB]

Problem

Currently you need a HashMap lookup to access channels in clients. On the server these hashmaps are accessed for every packet received.

Solution

Use a Vec to store the channels so lookups are fast. Channels should be generated in monotonically increasing order, so in general there won't be any empty entries. Even if there are empty entries, at most 256 channels can be created, so the memory cost of bad channel assignment is not significant.

Follow-up

• Reading packets requires a HashMap lookup for the target client. This is inefficient if a client has multiple packets, so a mutating iterator is preferable. Even better would probably be a global mutating iterator that traverses the client map.

[UkoeHB]

But I would probably remove channel_id from ChannelConfig and automatically assign it based on the index. This way we ensure that channels have sequenced IDs.

That would be a breaking change, this is just a refactor PR.